It is well-known that certain areas in an aircraft contain both a potential source of ignition and potential leakage of flammable liquid and/or vapor. In some of these areas, it is not possible to separate the potential ignition sources and any such leakage. Areas in commercial aircraft in which this separation cannot be accomplished are defined as fire zones and are required by the Federal Aeronautics Administration (FAA) to be separated from the rest of the aircraft by "fireproof" firewalls. Under FAA regulations, "fireproof" means able to withstand exposure to heat and flame at least as well as steel, or able to withstand exposure to a 2,000 degree F. flame for fifteen minutes without flame penetration. Designated fire zones include the regions in which each engine, auxiliary power unit, fuel-burning heater, and other combustion equipment intended for operation in flight are located. For example, the combustion, turbine, and tailpipe sections of turbine engines must be isolated from the rest of the aircraft.
In order to meet the FAA requirements, composite structures in engine nacelle and auxiliary power unit high temperature environments must be provided with flame and thermal protection. Known methods for providing such protection involve the use of nonstructural devices to shield the composite material structure. The methods presently in use include the application of a spray-on coating to the surface to be protected and the provision of insulation in the form of a separate blanket in front of the surface to be protected.
These methods have serious drawbacks since they tend to add to the cost of the aircraft, they add to the weight of the aircraft, and they are relatively difficult and expensive to maintain. Spray-on coatings are subject to cracking and peeling and therefore must be repaired or replaced fairly frequently. In addition, spray-on coatings are relatively difficult and time-consuming to apply and to inspect, adding to installation costs and further adding to maintenance costs. Separate blanket insulation systems add extra weight to the aircraft, consume valuable space in the aircraft, and are fairly costly to produce and install. In addition, in known blanket insulation systems the blanket is generally adhesively bonded and/or mechanically fastened to the structure being protected. The adhesive bonds are subject to peeling problems which add to the cost of maintenance and detract from the reliability of the protection provided. The mechanical bonds add to the weight of the aircraft, add to the cost of the aircraft by requiring more parts and more complicated installation procedures, and require special precautions to ensure against any discontinuity in the protection provided.
Each of the following U.S. patents discloses an apparently load-bearing wall or panel that is itself fire and/or heat resistant: No. 3,046,170, granted July 24, 1962, to H. A. Toulmin, Jr.; No. 3,106,503, granted Oct. 8, 1963, to B. M. Randall et al; No. 3,122,883, granted Mar. 3, 1964, to E. Terner; and No.3,364,097, granted Jan. 16, 1968, to J. B. Dunnington. Toulmin, Jr. discloses laminates of metal plated glass fibers. Randall et al disclose honeycomb panels of paper and asbestos with a flame and heat resistant cementitious coating. Terner discloses a heat resistant wall structure for rocket motor nozzles and the like. The wall has an outer steel portion, intermediate laminations of a refractory material such as graphite, insulating layers of material such as silica or quartz between these laminations, and an inner vented layer of a ceramic or metallo-ceramic material. Dunnington discloses a fire resistant panel consisting of a paper honeycomb filled with a fire retardant and having wood face sheets.
U.S. Pat. No. 4,302,496, granted Nov. 24, 1981, to J. G. Donovan, discloses a fire resistant waterproof fabric for tent walls and the like. The fabric has inner and outer woven flame resistant plies and a middle ply of a hydrophobic, moisture vapor permeable material.
The patent literature includes a number of examples of approaches to providing fire and/or thermal protection in the form of a barrier that may be attached to a load-bearing structure. U.S. Pat. No. 3,799,056, granted Mar. 26, 1974, to P. Colignon, and U.S. Pat. No. 4,037,006, granted July 19, 1977, to F. W. Roberts et al, disclose barriers that are mechanically attached to a structure. Colignon discloses thermal insulation for use between the heat shield and the body of a space vehicle. The insulation includes an outer thin metal sheet, an intermediate layer of insulation filling having a number of spaced refractory screens, and an inner layer of foamed polyimide. Roberts et al. disclose an insulating panel for use in building construction. The panel consists of a gypsum board with a layer of thermoplastic foam formed thereon and sheets of flame retardant material on the top and sides of the foam.
Thermal insulation that is adhesively bonded onto a structure is disclosed in U.S. Pat. Nos. 4,151,800, granted May 1, 1979, to R. L. Dotts et al; No. 4,232,620, granted Nov. 11, 1980, to M. Kurz; and No. 4,299,872, granted Nov. 10, 1981, to A. S. Miguel et al. Dotts et al disclose sheets of coated felt insulation that are adhesively attached to a spacecraft. Kurz discloses a thermal insulation of alternating layers of a mesh material such as nylon and a plastic film that are stitched together and then bonded to a structure with an adhesive. Miguel et al disclose a fire retardant thermal barrier to be adhesively bonded to the interior of an aircraft skin. The barrier is a honeycomb made from a material such as glass-phenolic filled with an intumescent material.
U.S. Pat. No. 4,310,587, granted Jan. 12, 1982, to P. M. Beaupre, discloses a fire resistant vapor barrier sheet for use with insulation inside building walls. The barrier includes a substrate of a material such as polyester having metalized surfaces and a layer of fire resistant radiation cured resin covering one of the metalized surfaces. The metalized surface opposite the resin is adhesively bonded to conventional insulation batting.
The following United States patents also disclose fire and/or thermal barriers: U.S. Pat. No. 3,203,849, granted Aug. 31, 1965, to H. S. Katz et al; No. 4,256,799, granted Mar. 17, 1981, to T. Ohashi et al; and No. 4,292,369, granted Sept. 29, 1981, to T. Ohashi et al. Katz et al disclose a heat shield for use in environments such as in aircraft and missiles. The shield has alternating layers of insulating material and heat conducting material, which layers are adhesively bonded together. The two Ohashi et al patents disclose fireproof laminates for use in building construction. The laminates include a foamed core with plies of fire resistant material on one or both sides that are attached through self-adhesion of the foam.
U.S. Pat. No. 4,122,203, granted Oct. 24, 1978, to J. S. Stahl, discloses a fire and thermal protective coating for foam plastic panels. The coating is a synthetic resin having hydrated magnesium sulphate dispersed therein and is flowed or sprayed onto a foam plastic panel.
Separate blanket insulation systems of the type used in aircraft are well-known in the art and commonly made by providing a layer of insulation fill material with front and back face sheets. Fibrous ceramic insulation fill has been available for a number of years. Such insulation was initially provided with stainless steel face sheets. Because of their weight, the stainless steel face sheets had to be very thin and thus had the serious disadvantage of being easily punctured. Quartz face sheets have been used, but have the disadvantage of being susceptible to crystallization and embrittlement at about 1300.degree. F. Fire conditions in aircraft generally produce surface temperatures in the range of 1600.degree. to 1900.degree. F.
The problems associated with stainless steel and quartz face sheets were solved when woven ceramic fabrics unaffected by temperatures up to about 2600.degree. F. became available. The Hexcel Corporation of Dublin, Calif. markets a blanket with front and back face sheets of a woven ceramic fabric sold by Hexcel Corporation under the trademark Nextel. This blanket has been used in DC-10 thrust reversers and has been FAA certified by fire testing. In such blankets, the face sheets are commonly attached to the insulation fill by sewing the three layers together.
Proposals have been made to attach the insulation and two face sheets to partially cured layers of graphite epoxy by sewing the insulation and face sheets and the graphite epoxy layers together. The curing process would then be completed. Since the graphite epoxy would be partially cured before being sewn to the blanket, the epoxy matrix would flow only slightly in the final curing process and would not be absorbed into the insulation through the ceramic fabric face sheet. If it were desired to attach such a sewn together product to further layers of fiber reinforced composite material, either before or after final curing, adhesives or mechanical fasteners would be required since the graphite epoxy sewn to the blanket would already have been cured to a point at which the epoxy matrix would flow only slightly.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative the prior art.